1. Field of the Invention
The invention relates generally to cross-linkable linear polymers containing carborane, siloxane, and acetylene groups, precursors thereof, and elastomers, plastics, and ceramics made therefrom.
2. Description of the Prior Art
The quest for high-temperature elastomers has been an evolving endeavor. An ever-increasing demand for such materials, especially in the aerospace industry, has served as a catalyst for continuing research efforts. The desired properties of such elastomers include long-term thermal, thermo-oxidative, and hydrolytic stabilities to temperatures approaching 400° C., and flexibility to temperatures as low as −50° C. Poly(siloxane)s are known to exhibit good high temperature resistance and excellent elasticity at low temperatures. Their elasticity is attributed to the pronounced conformational flexibility of their —Si—O—Si— backbone chain and to the ease of rotation around their Si—O bonds.
The thermal and oxidative properties of poly(siloxane) systems have been further improved by the incorporation of carboranes in such systems. The high chemical, thermal, and oxidative stabilities of carboranes can make the resultant poly(carborane-siloxane) systems even more resilient at high temperatures. The carboranes impart good protection against oxidative degradation. Furthermore, it has been seen that the incorporation of acetylenes into the backbones of poly(carborane-siloxane) systems improves the ability of these systems to retain their mass at very high temperatures. This is due to the acetylene-imparted ability to generate cross-linked centers, thereby reducing the preferences for skeletal cleavage of the backbone.
Polymers, ceramics, and precursors containing carborane, siloxane, and acetylene groups are disclosed in U.S. Pat. Nos. 5,272,237; 5,292,779; 5,348,927; 5,483,017; 5,756,629; 5,780,569; 5,932,335; 5,969,072; 5,981,678; 6,187,703; 6,225,247; and 6,265,336, all incorporated herein by reference.
U.S. Pat. No. 5,348,917 to Keller et al. discloses a carborane-siloxane-acetylene polymer having the formula:
This polymer is made by reacting 1,4-dilithio-1,3-butadiyne with a bis(chlorosiloxy)carborane. The polymer can be cross-linked between acetylene groups to a thermoset and pyrolyzed to a ceramic.
U.S. Pat. No. 5,780,569 to Keller et al. discloses a copolymer containing the above repeating unit and repeating units containing siloxyl and acetylene groups, as in the formula:
This polymer is made by reacting 1,4-dilithio-1,3-butadiyne with a bis(chlorosiloxy)carborane and a dichlorosiloxane. U.S. Pat. No. 5,483,017 to Keller et al. discloses that the polymer can be cross-linked between acetylene groups to a thermoset and pyrolyzed to a ceramic.
These polymers can have favorable thermal and oxidative properties. The acetylene groups aid in maintaining the polymer's shape and mass at very high temperatures. However, these polymers have a relatively high number of acetylene groups versus siloxane and carborane groups. This results in a high degree of cross-linking that inhibits the natural flexibility of the Si—O bonds. These cross-linked polymers are generally thermosets.
There is need for an elastomer having favorable thermal and oxidative properties. Such an elastomer may be useful in a variety of high temperature applications as insulation for electrical wire and as engine components. Polyimides are currently used as high temperature insulation; however, they readily decompose at 400° C. in air.